Kinetics of vinyl fluoride polymerization

Vinyl fluoride was polymerized, at low temperatures and pressures, by photo- and γ-ray initiation. The kinetics of this heterogeneous polymerization reaction was studied and a general scheme was proposed to explain the experimental results. The scheme is based on the assumption that the main site of the polymerization is in the “polymer phase.” The solvent used determines the rate of the reaction by its effect on the swelling of the polymer. The degree of polymerization is determined mainly by transfer reactions.     

Chiral phosphite-oxazolines: a new class of ligands for asymmetric Heck reactions

[chemical reaction: see text]. A series of phosphite-oxazoline ligands, derived from readily available D-glucosamine, have been used for the first time in the palladium-catalyzed Heck reaction of several substrates with high regio- and enantioselectivities (ee's up to 99%) and improved activities in standard conditions.     

Palladium-diphosphite catalysts for the asymmetric allylic substitution reactions

[structure: see text] We have designed a series of diphosphite ligands to study the effect of the backbone, the size of the chelate ring, and the substituents of the biphenyl moieties and to determine the scope of this type of ligand in the Pd-catalyzed asymmetric substitution reactions of different types of substrates. Good-to-excellent activities and enantioselectivities have been obtained for disubstituted linear substrate 11 (TOF's up to >2000 mol x (mol x h)(-1), ee values up to 99%) and cyclic substrate 14 (TOF up to 285 mol x (mol x h)(-1), ee values up to 92%). However, these ligands are inadequate for the Pd-catalyzed allylic alkylation of monosubstituted linear substrates because they provide low enantioselectivities.     

Synthesis of size-controlled cobalt ferrite particles with high coercivity and squareness ratio

Cobalt ferrite particles with diameters ranging from a few micrometer to about 15 nm were synthesized using a modified oxidation process. The fine control of the particle size was achieved by introducing various concentrations of Fe(3+) ions at the beginning of the reaction. Among the particle sizes obtained by using this method, particles with a grain size of about 36 nm showed a magnetization (M(s)) of 64 emu/g and a maximum coercivity (H(c)) of 2020 Oe at room temperature. The corresponding squareness ratio was found to be 0.53.     

Application of thermospray liquid chromatography-mass spectrometry to the simultaneous quantification of tracer concentrations of isotopically labelled carbamazepine epoxide and steady-state levels of carbamazepine and carbamazepine epoxide

A thermospray high-performance liquid chromatography-mass spectrometry method for the separation and quantification of tracer concentrations of isotopically labelled carbamazepine epoxide ([15N, 13C]CBZE) in the presence of steady-state levels of the anticonvulsant carbamazepine (CBZ) and its epoxide metabolite (CBZE) has been developed. The technique does not require derivatization, demonstrates little or no thermal degradation of the analytes, provides increased specificity not available from conventional high-performance liquid chromatography, and has a detection limit of 500 pg for CBZE on-column. The method, incorporating d4-CBZ and d4-CBZE as internal standards, allows precise and accurate determination of the analytes with good reproducibility and stability.     

Evaluating polarizable potentials on distributed memory parallel computers: Program development and applications

The efficient evaluation of polarizable molecular mechanics potentials on distributed memory parallel computers is discussed. The program executes at 7–10 Mflops/node on a 32-node CM-5 partition and is 19 times faster than comparable code running on a single-processor HP 9000/735. On the parallel computer, matrix inversion becomes a practical alternative to the commonly used iterative method for the calculation of induced dipole moments. The former method is useful in cases such as free-energy perturbation (FEP) simulations, which require highly accurate induced dipole moments. Matrix inversion is performed 110 times faster on the CM-5 than on the HP. We show that the accuracy which is needed for FEP calculations with polarization can be obtained by either matrix inversion or by performing a large number of iteration cycles to satisfy convergence tolerances that are less than 10^?6 D. © 1995 by John Wiley & Sons, Inc.     

Activity coefficients in mixed-electrolyte solutions at 25°C: Na-formate + NaCl system

Activity coefficients for NaCl in aqueous mixtures with Na-Formate were determined at 25°C from emf measurements for different total ionic strengths. At each total ionic strength studied, the measurements were carried out at different ratios of the Na-Formate to NaCl ionic strengths. The experimental activity coefficients were fitted using the Harned equation and the treatments of Scatchard et al. and Pitzer et al. Finally, the excess Gibbs energy of these mixtures was also calculated.     

Classical “freezing” of fast rotations. A numerical test of the Boltzmann-Jeans conjecture

We study numerically a very simple model representing a classical planar molecule, with only translational and rotational degrees of freedom, which collides with a fixed wall. On this model we test numerically an old conjecture by Boltzmann and Jeans, according to which the rate of the energy exchanges between the translational and the rotational degrees of freedom, due to collisions, decreases exponentially with the angular velocity of the molecule, giving rise to a purely classical phenomenon of freezing of fast rotations. Our results are in full agreement with the Boltzmann-Jeans conjecture. More precisely, we find that for each collision the average on the initial phase of the energy exchange, and the fluctuation, follow two different exponential laws; this fact turns out to have a rather delicate role in the approach of statistical equilibrium. A discussion of the numerical accuracy—which is rather high, since we are able to measure energy exchanges of one part over 10¹⁶—is also reported.     

Development of a headspace solid-phase microextraction-gas chromatography-mass spectrometry method for the identification of odour-causing volatile compounds in packaging materials

A method for the identification of volatile organic compounds in packaging materials is presented in this study. These compounds are formed by thermooxidative degradation during the extrusion coating process in the manufacture of packaging. Headspace solid-phase microextraction (HS-SPME) was used as sample preparation technique prior to the determination of the volatile organic compounds by gas chromatography-mass spectrometry (GC-MS). The effects of extraction variables, such as the type of fibre, the incubation temperature, the pre-incubation time, the size of the vial and the extraction time on the amounts of the extracted volatile compounds were studied. The optimal conditions were found to be: carboxen-polydimethylsiloxane 75 microm fibre, 5 min of pre-incubation time, 100 degrees C of incubation temperature, 20-ml vial, and 15 min of extraction time. The chromatograms obtained by HS-SPME and static headspace extraction were compared in order to show that the HS-SPME method surpasses the static headspace method in terms of sensitivity. Twenty-five compounds were identified including carbonyl compounds (such as 3-methyl-butanal, 3-heptanone or octanal), carboxylic acids (such as pentanoic acid or hexanoic acid) known as odour causing compounds and hydrocarbons (such as decane, undecane or dodecane). Finally, the method was applied to different packaging samples (one odour-unacceptable, two odour-acceptable, and three odourless samples) and to the raw materials in order to find out the odour-responsible volatile organic compounds and their source.